System and method for fractionation



April 1, 1969 D. M. BOYD SYSTEM AND METHOD FOR VFRCTIONATION Sheet FiledJuly 31, 1967 S :d ,Y MW E f5. M NM Ed 0 V./ .v T S Nw f r /0 A nl .S

/ @ESS sheet 2 of2 April l, 1969 D. M. BOYD SYSTEM lAND METHOD FORFRACTIONATION Filed July s1. 196'/ A TTRNEYS United States Patent O3,436,337 SYSTEM AND METHOD FOR FRACTIONATION David M. Boyd, ClarendonHills, Ill., assigner to Universal Oil Products Company, Des Plaines,Ill., a corporation of Delaware Filed July 31, 1967, Ser. No. 657,312Int. Cl. Cg 7/00;B01d 3/42; C07c 15/00 U.S. Cl. 208-92 13 ClaimsABSTRACT OF THE DISCLOSURE System and method for fractionating ahydrocarbon feed mixture such as one containing toluene and xyleneprecursors. The distillation of this mixture is controlled by samplingone product stream, converting the precursors therein to thecorresponding aromatic hydrocarbon via catalytic means, resolving theeffluent from the conversion zone via chromatographic means and using asignal generated from the chromatographic means to vary at least oneoperating condition, such as temperature, on the distillation column.

BACKGROUND OF THE INVENTION The present invention relates to a systemand method -for controlling uid separation processes. It particularlyrelates to a method for preparing selected feedstocks for a catalyticreforming process unit. It specically relates to a method for preparingselected feedstocks containing predetermined aromatic hydrocarbonprecursors for charge to a catalytic reforming process unit utilizingdistillation means having associated therewith analytical means fordetermining the presence of such aromatic hydrocarbon precursors.

In the production of petroleum and chemical products composed of amixture of components the basic and fundamental unit yoperation isdistillation. The petroleum rener, for example, uses distillation toseparate the components present in crude oil into selected fractionshaving desirable physical and chemical characteristics for furtherprocessing. For example, the fractions containing lubricating oilcomponents are initially separated via distillation means. The fractionshaving desirable motor fuel blending characteristics are also separatedby distillation means. In more recent times, the advent of catalyticreforming for both the production of gasoline motor fuel blendingcomponents and/or aromatic hydrocarbons, such as high purity benzene andtoluene and xylene, has demanded the use of distillation means forsegregatig from crude oil selected naphtha fractions which contain theproper precursors for the aromatic hydrocarbons. As those skilled in theart Well know, the catalytic reforming operation encompasses a Widevariety of chemical reactions, such as dehydrogenation, aromatization,cyclization, hydrocracking, and the like, in `order to concert thenaphtha feedstock by proper and selective molecular rearrangement intoproper aromatic hydrocarbons and/or motor fuel blending components.Traditionally, the naphtha fraction has been treated with hydrogen inorder to remove such deleterious contaminants, such as sulfur, and thencharged directly into the catalytic reforming unit. The effluent fromthe reforming unit is frequently separated by solvent extraction meansin order to recover therefrom high purity benzene, toluene, and thexylenes.

However, the aromatic hydrocarbon market has been subjected to differingdemands for specic aromatic hydrocarbons. In some instances there hasbeen a relatively high demand for benzene which has resulted in thedealkylation of the toluene formed in order to maximize benzeneproduction. In other instances the market for toluene has beendepressed, thereby causing a relatively Pice high inventory in themarket place of toluene, particularly n the market situations where thedemand for benzene has also stabilized.

Accordingly, it would be desirable to provide a method for preparingselected feedstocks for acatalytic reforming process -unit which wouldhave the flexibility of being adjustable according to market demand forthe various aromatic hydrocarbons which are produced therefrom. One suchmethod, of course, is the prefractionation of the traditional naphthafeed stock via distillation means roughly according to aromatichydrocarbon precursor type. However, in utilizing the prefractionationmeans, it was discovered that the control thereof was extremelydiflicult since the eiiciency of such a separation was not finallydeterminable until the effluent from the catalytic reforming processunit had been properly separated into its aromatic components. Thisenormous lag time between the opeartion of the prefractionation sectionand the separation of the catalytic reforming efuent into aromatichydrocarbons has presented extreme diiculties to the petroleum industry.For example, in a toluene depressed and benzene stabilized market itwould lbe desirable not to charge the toluene precursors to a catalyticreforming unit. However, if the catalytic reforming unit did produce anextraordinary large amount of toluene, the information necessary tocorrect the prefractionation section desirably is generally too late forproper control since variations in naphtha fedstock could easily havecaused different results to be taking place in the catalytic reformingunit during this analytical determination. In other Words, before anupset on the prefractionation section could be determied, a largev-olume of undesirable material had already been processed through thecatalytic reforming proces unit, thereby effecting considerable costs onthe operation as a whole.

SUMMARY OF THE INVENTION It s therefore an object of this invention toprovide a system and method for controlling fluid separation processes,particularly the distillation process.

It is another object of this invention to provide a method for preparingselected feedstocks for a catalytic reforming process unit.

Therefore, according to the practice of this invention, there isprovided a method for preparing selected feedstock for a catalyticreforming process unit which comprises: (a) introducing a hydrocarbonmixture comprising precursors for single ring aromatic hydrocarbons intoa separation zone under conditions suflicient to separate said mixtureinto a relatively light precursor concentrate stream, a relativelyintermediate precursor concentrate stream, and a relatively heavyprecursor concentrate stream; (b) passing a portion of a selectedprecursor concentrate stream through a miniatur-ized conversion zonemaintained under conditions suflicient to convert precursors in saidselected concentrate stream into single ring aromatic hydrocarbons; (c)resolving the hydrocarbon effluent from said conversion zone into bandsof identifiable aromatic hydrocarbons using chromatographic means; (d)obtaining from said chromatographic means a signal quantitativelycorrelatable to a predetermined single ring aromatic hydrocarbon; (e)employing said signal to vary said separation zone conditions inresponse to said quantitative measurement, thereby maintaining theamount of at least one precursor in said selective concentrate stream ata predetermined value; and (f) removing from said separation zone theremaining portion of the concrete stream of Step (b) as the selectedfeedstock for a catalytic reforming process unit.

Another embodiment of the invention includes the method hereinabovewherein said single ring aromatic hydrocarbons comprise benzene,toluene, and xylene and wherein said light precursor is for benzene,said intermediate precursor is for toluene, and said heavy precursor isfor the Xylenes. A

Still another embodiment of this invention includes the methodhereinabove wherein said signal of Step (e) varies a temperature of theseparation zone.

A still further embodiment of this invention provides a system forcontrolling a distillation column which cornprises in combination adistillation column, means for introducing a feed mixture into saidcolumn, means for withdrawing an overhead product fraction from the topof the column, means for withdrawing a heavier product fraction from thecolumn, means for controlling the temperature in an upper portion ofsaid column, sampling means for at least one of said product fractions,means for converting a component in said sample fraction to apreselected identifiable compound, means for analyzing the product fromsaid converting means for said identitiable compound, means forgenerating a signal from said analyzing means quantitativelycorrelatable to said identiliable compound, and means for employing saidsignal to vary said temperature control means responsive to saidquantitative measurement.

Thus, one feature of the present invention involves the use ofanalytical means for controlling a prefabrication system which ispreparing feedstock for a subsequent conversion operation. Theanalytical means in essence further involves the use of a miniaturizedconversion zone which is substantially identical in function to thesubsequent commercial conversion operation for which the selectedfeedstock is being prepared. The effluent from this miniature operationis characterized by chromatographic means which generate a proper signalwhich is utilized in varying the operating conditions in theprefractionation section according to the quantitative results obtainedby the overall analytical means. It was found that by operating in thismanner the lapsed time between sampling and adjustment of conditions wasconsiderably lessened. Frequently, the practice of this inventionenabled changes to be made in the prefractionation section in a shortertime as from 5 minutes to 30 minutes whereas previous operations wouldhave required several hours or more for determining whether or notcorrective measures were needed in the prefractionation system.

The selection of the proper precursor for use in controlling thedistillation means for preparing the selected feedstock for catalyticreforming process units can, of course, be varied widely, depending uponthe desires of the Iener. Preferably, however, for the situation ofdepressed toluene market, the analysis is performed on the xyleneprecursor stream and is correlated to the presence of toluene in suchprecursor stream. A convenient means of control is to adjust theoperating conditions in the prefractionation zone lto produce less thanby volume toluene in the xylene precursor stream. More preferably, lessthan 5% of toluene is an acceptable target. It is to be understood,however, that the quantitative measurement which is correlated to aparticular precursor can be on Xylene or any other measurement. From theteachings presented herein, it is equally feasible for the analyticaltechnique to be performed on the toluene precursor stream, such that aproper Xylene precursor stream is also produced. Any other variationsobvious to those skilled in the art from the teachings presented hereinis broadly encompassed in the concept of the present invention.

While this invention has been described with particular reference forthe prefractionation of a benzene, toluene, and Xylene precursor stream,it will be obvious to those skilled in the art that the concepts of thepresent invention are equally applicable to any other environment whichis of the precursor type. In other words, the concepts of this inventioncan be utilized to prepare selected feedstocks for any conversion zonewherein the sample is actually 4 converted in a miniaturized conversionzone and the results of this sample conversion are more or lessimmediately translated into the control of the equipment preparing thefeedstock for such conversion zone.

As previously indicated, the present invention is uniquely applicable tothe preparation of a selected feedstock for a catalytic reformingprocess unit. Therefore, the miniaturized conversion zone as stated inthe claims embodies a catalytic reforming reactor containing typicallyplatinum catalyst and utilizing conventional operating conditions whichare also used in the commercial unit which subsequently follows theprefractionation operation. One advantage in operating in this manner isthat the hydrogen produced in this miniaturized reactor can also be usedas the carrier gas in the subsequent chromatographic separation means.Another advantage, of course, is that the catalyst in the miniaturizedreactor does not require regeneration. Since it is of such small scale,any deactivation of the catalyst can be corrected by the addition offresh catalyst, or if desired a series of miniaturized reactors can beused in swing reactor fashion in order to process the proper sample.

The chromatographic separation means utilized in this analyticaltechnique is well known to those skilled in the art. This subject isdiscussed in detail in the publication Gas Chromatography, by A. I. M.Keulemans (Rinehold Publishing Corporation, 1957). Therefore, a detailedpresentation of gas chromatography methods will not be incorporatedherein. Those skilled in the art are referred to such notablepublications for the details of this separation technique.

Briefly, chromatographic separation means includes introducing a gaseoussample into a column which is packed with suitable separatory material.The components of the sample are separated by sweeping the column with acarrier gas so that the components are eluded from the separatorymaterial and emerge from the column in bands or slugs of materialshaving similar elution velocities roughly determined by their well knownpartition coeiiicients. A detecting device is used to sense thecomposition of the bands of components as they emerge from the columnand a quantitative and qualitative determination of the sample may beobtained.

In the present invention, the detection device also generates a signalwhich is amplified and correlatable to the desirable aromatic precursoror actually the aromatic hydrocarbon, and which is passed into oneintegrator which is adaptable for the specific aromatic hydrocarbon. Thesignal is also passed into another integrator which is adaptable for thetotal aromatics produced in the miniaturized reactor. The signals fromthe integrators are passed into a suitable dividing circuit whichcalculates the percent of the specific aromatic hydrocarbon present inthe sample stream. Since the chromatographic means is a batch operationand therefore any signal therefrom is intermitten-t, the informationfrom the integrators and dividing circuits are stored in suitable memoryblocks from which a combined signal is transmitted to a conventionalrecorder-controller which activates control valves for varying theselected operating conditions on the prefractionation zone.

All of the specific instruments which are utilized in this broadanalytical device are conventional and well known to those skilled inthe art. As non-limiting examples of this equipment the chromatographycolumn may be a Beckman Model 320-C Process Gas Chromatograph, theintegrators may be conventional analog electronic integrators whichproduce a current output proportional to the length of time andamplitude of the applied signal. At this point it should be noted thatthe purpose of the integrator is actually to record the cumulative areaof the chromatogram at timed intervals, preferably, about every l0 to l5seconds, or a similar digital system could be employed. Therecorder-controller, of course, is conventional and is preferably arecording potentiometer. Frequently, the signal from the detection unitmust be amplified and preferably an amplifier is used. The actualamplications developed will vary, of course, depending upon the type ofrecorder and/or integrator employed. However, some method is requiredfor amplifying the millivolt signal from the -chromatograph bridge inorder to feed it to the integrators and recorder. Thus, the amplifiermay be of any suitable type available commercially.

The packing material utilized in the chromatographic column can be anyof the inert stationary type. Advantageously, crushed rebrick ordiatomaceous earth is used as an inert solid support for a partitioningliquid, such as diethylene glycol succinate polymer which will resolvethe various components in the feed to the chromatographic column intobands of its individual components for detection. Preferably, crushedfirebrick of about 40 to SO mesh is employed for most purposes.

The carrier gas employed in sweeping or eluting the components of thefeedstock to the chromatographic column may he of any of theconventional carrier gases which are not reactive with the samplemixture or the materials of the apparatus and which do not produce aresponse in the detector device. Among the carrier gases generallysuitable for this are hydrogen, helium, nitrogen, air, carbon dioxide,argon, etc., with, as previously mentioned, hydrogen being the preferredcarrier gas for the practice of this invention.

Those skilled in the art are familiar with the difficulties of utilizingan intermittent signal for process control. Exemplary of solutions foundby those skilled in the art may be understood with reference to U.S.Patent No. 2,994,646 to L. D. Kleiss and U.S. Patent No. 3,009,864 to R.D. Webb. Both of these patentees address themselves to the handling ofthe intermittent signal resulting from the detection device accompanyingchromatographic separation means. These solutions and others known tothose skilled in the art may be found useful for incorporation in thesystem and method of the present invention, although such devices itmust be pointed out, are not absolutely necessary for the practice ofthis invention.

The invention may be more fully understood with reference to theaccompanying drawings wherein:

FIGURE 1 is a schematic representation of prefractionation means forpreparing a selected feedstock `for a catalytic reforming process unit,and

FIGURE 2 is a more detailed schematic representation of theinstrumentation involved in the system for controlling the distillationcolumn.

DECRIPTION OF THE DRAWINGS Referring now to FIGURE l, a suitable naphthafeedstock containing aromatic hydrocarbon precursors enters the systemvia line and is passed into distillation column 11. Operating conditionsare maintained in column 11 sufficient to produce a relatively lightprecursor concentrate stream which is withdrawn via line 12 and leavingbehind an aromatic hydrocarbon precursor bottoms stream whichsubsequently will be sep arated into relatively intermediate concentratestreams and relatively heavier concentrate streams and which iswithdrawn from column 11 via line 16.

The material in line 12 is passed into a further distillation column 13wherein light hydrocarbons and normally gaseous hydrocarbons arewithdrawn via line 14 and a relatively light precursor concentratestream comprising, in this case, as an example, benzene precursors, iswithdrawn via line 15 for charge to a catalytic reforming process unit,if desired. The material in line 16 contains among other thingsprecursors for toluene and precursors for xylene. Illustrative of theoperation of the present invention, this material is passed intodistillation column 17 wherein it is desired to split from the feed thetoluene precursors so that a concentrate of xylene precursors will beavailable as charge to the subsequent catalytic reforming operation.Accordingly, suitable operating conditions are maintained indistillation column 17, preferably controlled by temperature, utilizingTRC 21, the control of which is more fully explained hereinafter. Thetoluene precursor stream is removed via line 18 and utilized, forexample, as a motor fuel blending component, A residuum stream iswithdrawn from distillation column 19 and may also be used in motor fuelblending.

The desired xylene precursor stream is withdrawn from column 17 via.line 20 and passed preferably in conjunction with the material in line15 into a subsequent catalytic reforming process unit operating inconventional manner.

According to this invention, a sample of the material in line 20 ispassed via line 22 through suitable sample valve means (not shown),admixed with relatively pure hydrogen gas from line 23, and passed intoreactor 24 which contains platinum on -alumina catalyst. Properoperating conditions are maintained in reactor 24 to convert anyaromatic hydrocarbon precursors in the sample into the respectivearomatic hydrocarbon. The hydrocarbon effluent is removed from reactor24 via line 25 and passed into chromatographic column 26 which operatesin conventional manner under conventional conditions utilizing hydrogenas the carrier gas. Typically, chromatographic column 26 may be packedwith a substrate of diethylene glycol succinate on 42-60y mesh crushedfirebrick. As the bands of components :are eluted from column 26, theyare passed via line 27 through detector circuit 28 which containssufficient devices for generating a proper signal quantitativelycorrelatable, for example, on the toluene content of the material in thesample obtained in line 22. This signal is passed via appropriatecircuit 29 into recorder-controller 30 which activates TRC 21 accordingto the quantitative measurement obtained from the detector circuit 28.TRC 21 can, of course, activate control valves on the reflux stream (notshown) to column 17 or can activate control valves on the reboilersystem, if any, (not shown) on column 17. It should also be noted atthis point that recorder-controller 30 can, of course, vary otheroperating conditions in column 17, such as pressure and/or reflux ratio,etc. The function of the information of recorder-controller 30 is toproperly adjust the operating conditions in distillation column 17 sothat the predetermined value of xylene precursors is ultimately removedfrom the system in line 20'. As those skilled n the art know, theinitial portion of the detector circuit 28 contains a detection device,such as la thermal conductivity cell or an ionization detector or otherdevices well known to those skilled in the art. It may be advantageousin the practice of this invention to use a hydrogen flame ionizationdetector since hydrogen gas is already available in the analyticalsystem.

Referring now to FIGURE 2, it should be noted that the same numeralshave been utilized to denote the same items as was used in FIGURE 1 forease of analysis. As previously mentioned, the feed in line 16 containsaromatic hydrocarbon precursors for both toluene and xylene. Thismaterial is passed into distillation column 17 which is maintained underdistillation conditions and controlled by temperature, utilizing TRC 21.The toluene precursor stream or relatively intermediate precursorconcentrate stream is removed from column 17 via line 18. A residuumstream is removed from column 17 via line 19. The material in lines 18and 19 are preferably both utilized as gasoline blending components. Axylene precursor concentrate stream or relatively heavy precursorconcentrate stream is withdrawn from column 17 via line 20 and passedpreferably to catalytic reforming process units, not shown. A sample ofthe material in line 20 is obtained via line 22 through suitable samplevalves, not shown, admixed with hydrogen from line 23, and passed intocatalytic reforming reactor 24 which is 7 a miniaturized version of thepreviously mentioned subsequent catalytic reforming process unit.Reactor 24 contains platinum or alumina catalyst and is operated underconditions of temperature and pressure substantially the same as that ofsaid subsequent process unit. The efliuent withdrawn at a temperature ofabout 306 F. Operating under these conditions, the following separationwas made with TRC 21 being controlled by the inventive method of thepresent invention at a temperature of 240 F.

TABLE I Line No Component:

from reactor 24 is passed via line 25 into chromatographic column 26wherein the aromatic hydrocarbon components in the feed are resolvedinto individual component bands and eluted with hydrogen gas. The bandsas they are removed from column 26 are passed via line 27 into detectingdevice 28 which in particular contains detector 31 which may be of thethermal conductivity type. The sample having passed through detector 31may be passed via line 43 back into the main stream of xylene precursorconcentrate in line or may be discarded. The signal generated bydetector 31 is passed via lead 32 into amplifier 33. The signals arepassed via lead 34 in at least two fashions. The signal containing thetoluene component is passed into integrator 36 and subsequently via lead37 into dividing circuit 38. Another signal which is correlatable to thetotal aromatic content of the sample is passed via lead 39 intointegrator 40 from which this total integrated signal is passed via lead41 into dividing circuit 38 for the calculation of the amount of, say,toluene in line 22. This information is stored in a suitable memorydevice 42 and subsequently passed via lead 29 into recorder-controller30. Recorder-controller 30 then varies TRC 21 which in turn varies theoperation of column 17 responsive to the quantitative measurementobtained from detecting circuit 28. It is to be noted that since theinventive device and method is responsive to the ratio of, say, toluene,to total aromatic hydrocarbons, the catalytic convertor does not have tobe 100% eiiicient in converting precursors to aromatics.

The following example will illustrate the preferred embodiment of theinvention.

Example A commercial scale prefractionation zone arranged according tothe configuration of FIGURE 1 was operated by charging a virgin naphthastream into the system via line 10, as previously mentioned.Distillation column 11 was operated under a slightly superatmosphericpressure of approximately 10 to 15 pounds, an overhead temperature of165 F., and a bottoms temperature of about 360 F. Distillation column 13was operated under an imposed pressure of about pounds, an overheadtemperature of about 150 F., and a bottoms temperature of about 280 F.Distillation column 17 was operated under an imposed pressure ofapproximately 15 p.s.i.g., an overhead temperature of 241 F., a bottomstemperature of about 370 F., and the side-cut stream in line 20 was Thecontrol feature of the present invention embodied in the system sampledthe material in line 20 according to conventional chromatographytechniques which included, preferably, the use of a sample chargingvalve although a serum cap and syringe, a bulb and crushing apparatus,or the like, could also be used. The sample Was heated by suitablemeans, such as an electrical resistance heater, in order to vaporize thesample upon introduction into the chromatographic column. On the otherhand, the material from the sample reactor 24 may be passed withoutcondensation directly in its vapor state via line 25 intochromatographic column 26. However, depending upon the sophistication ofthe technique of sampling, it may be desirable to condense the effluentfrom reactor 24 and then use the conventional sampling means in order toproperly charge chromatographic column 26. The sample size may varyconsiderably; for instance, 3 microliter sample may be used or more. Thepredetermined value for the presence of toluene after conversion in theminiature reactor was chosen at 8% toluene. In other words, it was thedesire to charge a material in line 20 to the catalytic reformer whichcomprised precursors which would yield 92% xylene and only 8% toluenefrom the commercial unit. Satisfactory control of distillation column 17was achieved in that the lapsed time between the determination ofpercent toluene in line 20 to a change of operating conditions was onlyapproximately 20 minutes.

Therefore, the present invention provided a facile and economical mannerof controlling the prefractionation section which prepared a selectedfeedstock for a catalytic reforming process unit. It should be notedthat the material in line 20 contained a considerable variety of C7 toC9 hydrocarbons and therefore on the basis of hydrocarbon type it wouldhave been extremely diicult to determine from the breakdown shown in thetable exactly how much xylene would have been produced from this kind ofa feedstock. Utilizing the miniaturized reactor technique coupled withchromatographic detection means enabled the refiner to achieve excellentcontrol over the subsequent catalytic reforming process unit in terms ofmaximizing benzene and xylene production while minimizing or excludingentirely any significant production of toluene.

Preferred embodiment lt can be seen from the description of theinvention so far that the present invention in its preferred embodimentprovides a method for preparing selected feedstock for a catalyticreforming process to produce aromatic hydrocarbons which comprises thesteps of: (a) introducing a hydrocarbon mixture containing precursorsfor benzene, toluene, and xylene into a first distillation zone andwithdrawing therefrom an overhead stream comprising benzene precursorsand a bottoms stream comprising precursors for toluene and precursorsfor the xylenes; (b) passing said bottoms stream into a seconddistillation zone and withdrawing therefrom a distillate streamcomprising precursors for toluene and a heavier boiling fractioncomprising precursors for -the xylenes, said second zone beingmaintained under distillation conditions including temperature sufcientto maintain the concentration of toluene precursors in said heavierboiling fraction at a preselected value; (c) passing a minor portion ofsaid heavier boiling fraction through a conversion zone maintained underaromatization conditions sut`n`cient to convert precursors therein intoaromatic hydrocarbons comprising toluene and the xylenes; (d)introducing the aromatic-containing eluent from the conversion zonetogether with hydrogen carrier gas into chromatographic means underconditions suicien-t to resolve said eflluent into bands of saidaromatic hydrocarbons; (e) detecting said bands and obtaining from suchdetection a signal quantitatively correlatable to at least one of saiddetected bands; (f) employing said signal to vary the distillationtemperature of Step (b) in response to the quantitative determination ofStep (e); and (g) recovering from said second distillation zone theheavier boiling fraction as the selected feedstock to a catalyticreforming process.

Another preferred embodiment of the invention includes the preferredmethod hereinabove wherein said preselected value for the concentrationof toluene in said selected feedstock is less than by volume.

The invention claimed:

1. Method for preparing selected feedstock for a catalytic reformingprocess unit which comprises:

(a) introducing a hydrocarbon mixture comprising precursors for singlering aromatic hydrocarbons into a separation zone under conditionssuicient to separate said mixture into a relatively light precursorconcentrate stream, a relatively intermediate precursor concentratestream, and a relatively heavy precursor concentrate stream;

(b) passing a portion of a selected precursor concentrate stream througha miniaturized conversion zone maintained under conditions suicient toconvert precursors in said selected concentrate stream into single ringaromatic hydrocarbons;

(c) resolving the hydrocarbon eluent from said conversion zone intobands of identiliable aromatic hydrocarbons using chromatographic means;

(d) obtaining from said chromatographic means a signal quantitativelycorrelatable to a predetermined single ring aromatic hydrocarbon;

(e) employing said signal to vary said separation zone conditions inresponse to said quantitative measurement, thereby maintaining theamount of at least one precursor in said selected concentrate stream ata predetermined value; and

(f) removing from said separation zone the remaining portion of theconcentrate stream of Step (b) as the selected feedstock for a catalyticreforming process unit.

2. Method according to claim 1 wherein said concentrate stream of Step(b) is selected from the group consisting of said intermediate streamand said heavy stream.

3. Method according to claim 2 wherein said single ring aromatichydrocarbons comprise benzene, toluene, and xylene and wherein saidlight precursor is for benzene, said intermediate precursor is fortoluene, and said heavy precursor is for the xylenes.

4. Method according to claim 1 wherein said signal in Step (e) varies atemperature of the separation zone.

5. Method for preparing selected feedstock for a catalytic reformingprocess to produce aromatic hydrocarbons which comprises the steps of:

(a) introducing a hydrocarbon mixture containing precursors for benzene,toluene, and the xylenes into a rst distillation zone and withdrawingtherefrom an overhead stream comprising benzene precursors and a bottomsstream comprising precursors for toluene and precursors for the xylenes;

(b) passing said bottoms stream into a second distillation zone andwithdrawing therefrom a distillate stream comprising precursors fortoluene and a heavier boiling fraction comprising precursors for thexylenes, said second zone being maintained under distillation conditionsincluding temperature suicient to maintain the concentration of toluenepre- -cursors in said heavier boiling fraction at a preselected value;

(c) passing a minor portion of said heavier boiling fraction through aconversion zone maintained under aromatization conditions suicient toconvert precursors therein into aromatic hydrocarbons comprising tolueneand the xylenes;

(d) introducing the aromatic-containing eluent from the conversion zonetogether with hydrogen carrier gas into chromatographic means underconditions sufficient to resolve said effluent into bands of saidaromatic hydrocarbons;

=(e) detecting said bands and obtaining from such detection a signalquantitatively correlatable to at least one of said detected bands;

(f) employing said signal to vary the distillation temperature of Step(b) in response to the quantitative determination of Step (e) and (g)recovering from said second distillation zone the heavier boilingfraction as the selected feedstock to a catalytic reforming process.

6. Method according to claim 5 wherein said preselected value for theconcentration of toluene in said selected feedstock is less than 10% byvolume.

7. Method according to claim 6 wherein said signal of Step I(e) isquantitatively correlatable to the toluene band.

8. In a distillation process for separating precursors of aromatichydrocarbons into a rst fraction comprising precursors of relativelylight aromatic hydrocarbons and a second fraction comprising precursorsof relatively heavy aromatic hydrocarbons wherein said distillation isinuenced by temperature, the improvement which comprises passing sampleof one of said fractions into a miniaturized conversion zone underconditions sutlicient to convert precursors therein to aromatichydrocarbons, resolving the hydrocarbon effluent from said conversionzone into bands of identifiable aromatic hydrocarbons usingchromatographic means, obtaining from said chromatographic means asignal quantitatively correlatable to a preselected converted aromatichydrocarbon, and employing said signal to vary said distillationtemperature in response to the quantitative measurement therebymaintaining the amount of at least one aromatic hydrocarbon precursor insaid sampled fraction at a predetermined value.

9. Improvement according to claim 8 wherein said sampled fractioncomprises said second fraction and said preselected converted aromatichydrocarbons comprises said light hydrocarbons.

10. Improvement according to claim 9 wherein said first fractioncomprises precursors for toluene and said second fraction comprisesprecursors for the xylenes.

11. System for controlling a distillation column which comprises incombination, a distillation column, means for introducing a feed mixtureinto said column, means for withdrawing an overhead product fractionfrom the top of the column, means for withdrawing a heavier productfraction from the column, means for controlling the temperature in anupper portion of said column, sampling means for at least one of saidproduct fractions, means for converting a component in said sampledfraction to a preselected identifiable compound, means for analyzing theproduct from said converting means for said identifiable compound, meansfor generating a signal from said analyzing means quantitativelycorrelatable to said identifiable compound, and, means for employingsaid signal to vary said temperature control means responsive to saidquantitative measurement.

12. System according to claim 11 wherein said sampling means comprisesmeans for sampling said heavier product fraction.

selected identiable compound is the major constituent in said overheadproduct fraction.

References Cited UNITED STATES PATENTS 2,808,368 10/1957 Moser 208-1633,000,812 9/1961 Boyd 208-138 3,185,640 5/1965 Beavon 208-134 3,368,9662/1968 Borst et al. 208-351 10 HERBERT LEVINE, Primary Examiner.

U.S. C1. XR.

13. System according to claim 12 wherein said pre- 15 134; 260-668

